In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded, in part, on the development of a light source of a shorter wavelength. A wavelength reduction from a mercury lamp of i-line (365 nm) to a KrF excimer laser (248 nm) enabled the mass production process of 64 M-bit dynamic random access memory (DRAM, processing feature size 0.25 μm or less). For the fabrication of DRAM with a degree of integration of 256 M and 1 G or more, photolithography using ArF excimer laser light (193 nm) has been under active investigation. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 or greater is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, a F2 laser of 157 nm wavelength became a candidate. However, for the reasons that the scanner becomes expensive, the optical system must be altered, and the etch resistance of resist is low; the application of F2 lithography was postponed. The ArF immersion lithography was proposed as a replacement. Since then, efforts have been made for the early introduction thereof (see Proc. SPIE Vol. 4690 xxix).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water, and ArF excimer laser radiation is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724, 2003).
In the ArF lithography, acid-catalyzed chemical amplification positive working resist materials are used as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). While novolac resins and polyhydroxystyrene resins are used as the alkali-soluble base resin in the i-line and KrF lithography resists, they cannot be used in the ArF lithography because of their very strong absorption at a wavelength around 193 nm. Instead, studies were made on poly(meth)acrylate resins and resins comprising cycloaliphatic olefin such as norbornene as polymerized units, both using carboxyl groups as the alkali-soluble group (see JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198).
Of these, the poly(meth)acrylate resins are expected to reach a practical level because of easy polymerization. One of the poly(meth)acrylate resins proposed thus far is a poly(meth)acrylate resin having methyladamantyl groups as the acid labile group and lactone rings as the adhesive group as disclosed in JP-A 9-90637. Resins containing norbornyl lactone as the adhesive group were developed for enhancing etch resistance as disclosed in JP-A 2000-26446 and JP-A 2000-159758. A study is made to apply to the ArF lithography a copolymer of norbornene and a-trifluoromethylacrylate, which was the base polymer candidate for F2 resist, because it has high transparency around wavelength 193 nm and high etching resistance (see Proc. SPIE, Vol. 4345, p 273 (2001), for example).
Since the ArF immersion lithography carries out light exposure through water, there arise problems different from the conventional lithography. For example, if water left on the resist film after exposure penetrates into the resist film, it, even in trace amounts, can cause development defects. One of means proposed thus far for preventing water penetration is by forming a topcoat film on the resist film. Another effective means contemplated is by enhancing the water repellency of the resist film itself.
In order to enhance the water repellency of the resist film, the base resin must be rendered more water repellent. In general, resins can be made more water repellent by incorporating fluorine atoms. However, as the proportion of fluorine incorporated is increased, the resins becomes less adherent to substrates. It would be desirable to have a fluorinated resin which has high water repellency while maintaining good substrate adhesion.